Analysis of the anisotropy of excitons in pentacene single crystals using reflectivity measurements and electron energy-loss spectroscopy

Grobosch, M.; Schuster, R.; Pichler, Thomas; Knupfer, M.; Berger, H.

doi:10.1103/physrevb.74.155202

Cited by 18 publications

(23 citation statements)

References 54 publications

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“…S5 [18]), only limited by the size of BN. We note that the anisotropy of PL had only been indirectly observed in the highest quality pentacene single crystals by ellipsometry and electron energy-loss spectroscopy [29]. The PL of 1L and 2L samples composed of two prominent peaks, centred at 2.16eV and 2.29eV for 1L and 2.13eV and 2.25eV for 2L, as well as a small peak near 2.4eV ( Supplementary Fig.…”

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confidence: 95%

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Probing Carrier Transport and Structure-Property Relationship of Highly Ordered Organic Semiconductors at the Two-Dimensional Limit

et al. 2016

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One of the basic assumptions in organic field-effect transistors, the most fundamental device unit in organic electronics, is that charge transport occurs two-dimensionally in the first few molecular layers near the dielectric interface.Although the mobility of bulk organic semiconductors has increased dramatically, direct probing of intrinsic charge transport in the two-dimensional limit has not been possible due to excessive disorders and traps in ultrathin organic thin films. Here, highly ordered mono-to tetra-layer pentacene crystals are realized by van der Waals (vdW) epitaxy on hexagonal BN. We find that the charge transport is dominated by hopping in the first conductive layer, but transforms to band-like in subsequent layers.Such abrupt phase transition is attributed to strong modulation of the molecular packing by interfacial vdW interactions, as corroborated by quantitative structural characterization and density functional theory calculations. The structural modulation becomes negligible beyond the second conductive layer, leading to a mobility saturation thickness of only ~3nm. Highly ordered organic ultrathin films provide a platform for new physics and device structures (such as heterostructures and quantum wells) that are not possible in conventional bulk crystals. 3Organic field-effect transistors (OFETs) offer unique advantages of low cost, lightweight and flexibility and are widely used in electronics and display industry.While the mobility of bulk organic semiconductors has increased dramatically [1][2][3], an outstanding issue is to directly examine the structure-property relationship at the semiconductor-dielectric interface [4], where charge transport actually occurs [5][6][7].Ultrathin organic semiconductors down to few-nanometre thickness are often dominated by traps and disorders and far away from intrinsic transport regime [8][9][10].Another challenge in organic electronics is the development of layer-by-layer epitaxy with the precision similar to molecular beam epitaxy in their inorganic counterparts [11]. These challenges may be alleviated if molecular crystals are processed into large-area, highly crystalline monolayers. Such 2D form factor will also bring about new applications such as nanoporous membranes and insulating dielectrics [12,13].Several recent breakthroughs in various types of 2D organic materials such as polymers [14,15], oligomers [16] and covalent organic frameworks [17] have already shown great promises along this direction. However, one of the most fundamental questions regarding the nature of charge transport at the 2D limit has not been addressed. In this work, we study the benchmark molecule pentacene epitaxially crystallized on BN substrate because of its high mobility and simple structure to model. The highly clean system allows us to provide the first definitive scenario of how molecular packing and charge transport are modulated near the interface, without being dominated by extrinsic factors. Our results suggest the possibility of band-like transport...

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confidence: 95%

“…We carried out presumably tailored by the crystal symmetry. The direction of the highest (lowest) PL intensity was assigned to the a-axis (b-axis) of the pentacene crystals [29].…”

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confidence: 99%

Probing Carrier Transport and Structure-Property Relationship of Highly Ordered Organic Semiconductors at the Two-Dimensional Limit

et al. 2016

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“…In spite of extensive characterization of its electrical properties, the interpretation of electronic excitations in pentacene solids, in particular the nature of the lowest excitations, is still conflicting. While earlier work described them as mainly molecular Frenkel excitons without CT excitation contribution, 7,8 recent experimental studies using reflectivity measurements and electron energyloss spectroscopy 9,10 as well as sophisticated theoretical approaches 11 suggest CT-like excitonic contributions near the onset of absorption. Clarifying the CT contribution in the absorption onset has practical implications for organic photovoltaic devices, since CT exciton is generally considered as an important precursor in the creation of free charge carriers by light absorption.…”

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confidence: 99%

Observation of Frenkel and charge transfer excitons in pentacene single crystals using spectroscopic generalized ellipsometry

Qi¹,

Su²,

Bastjan³

et al. 2013

Applied Physics Letters

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“…S5 [18]), limited by the size of BN. We note that the anisotropy of PL had only been indirectly observed in the highest quality pentacene single crystals by ellipsometry and electron energy-loss spectroscopy [43]. The PL of 1L and 2L samples composed of two prominent peaks, centered at 2.16 and 2.29 eV for 1L, and 2.13 and 2.25 eV for 2L, as well as a small peak near 2.4 eV (Supplemental Material, Fig.…”

Section: Prl 116 016602 (2016) P H Y S I C a L R E V I E W L E T T Ementioning

confidence: 97%

“…S5, S6 [18]], presumably tailored by the crystal symmetry. The direction of the highest (lowest) PL intensity was assigned to the a axis (b axis) of the pentacene crystals [43]. Single-crystalline 1L with lateral size up to ∼60 μm has been observed (Supplemental Material, Fig.…”

Section: Prl 116 016602 (2016) P H Y S I C a L R E V I E W L E T T Ementioning

confidence: 98%

Precise Layering of Organic Semiconductors

Natelson

2016

Physics

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One of the basic assumptions in organic field-effect transistors, the most fundamental device unit in organic electronics, is that charge transport occurs two dimensionally in the first few molecular layers near the dielectric interface. Although the mobility of bulk organic semiconductors has increased dramatically, direct probing of intrinsic charge transport in the two-dimensional limit has not been possible due to excessive disorders and traps in ultrathin organic thin films. Here, highly ordered single-crystalline monoto tetralayer pentacene crystals are realized by van der Waals (vdW) epitaxy on hexagonal BN. We find that the charge transport is dominated by hopping in the first conductive layer, but transforms to bandlike in subsequent layers. Such an abrupt phase transition is attributed to strong modulation of the molecular packing by interfacial vdW interactions, as corroborated by quantitative structural characterization and density functional theory calculations. The structural modulation becomes negligible beyond the second conductive layer, leading to a mobility saturation thickness of only ∼3 nm. Highly ordered organic ultrathin films provide a platform for new physics and device structures (such as heterostructures and quantum wells) that are not possible in conventional bulk crystals. DOI: 10.1103/PhysRevLett.116.016602 Organic field-effect transistors (OFETs) offer unique advantages of low cost, light weight, and flexibility and are widely used in the electronics and display industry. While the mobility of bulk organic semiconductors has increased dramatically [1][2][3], an outstanding issue is to directly examine the structure-property relationship at the semiconductor-dielectric interface [4], where charge transport actually occurs [5][6][7]. Ultrathin organic semiconductors a few nanometers thick are often dominated by traps and disorders and far away from the intrinsic transport regime [8][9][10]. Another challenge in organic electronics is the development of layer-by-layer epitaxy with precision similar to molecular beam epitaxy in their inorganic counterparts [11]. These challenges may be alleviated if molecular crystals are processed into large-area, highly crystalline monolayers. Such a 2D form factor will also bring about new applications such as nanoporous membranes and insulating dielectrics [12,13]. Several recent breakthroughs in various types of 2D organic materials such as polymers [14,15], oligomers [16], and covalent organic frameworks [17] have already shown great promise in this direction. However, one of the most fundamental questions regarding the nature of charge transport at the 2D limit has not been addressed. In this work, we study the benchmark molecule pentacene, epitaxially crystallized on a BN substrate because of its high mobility and simple structure to model. The highly clean system allows us to provide the first definitive scenario of how molecular packing and charge transport are modulated near the interface, without being dominated by extrinsic factors. Our...

show abstract

Analysis of the anisotropy of excitons in pentacene single crystals using reflectivity measurements and electron energy-loss spectroscopy

Cited by 18 publications

References 54 publications

Probing Carrier Transport and Structure-Property Relationship of Highly Ordered Organic Semiconductors at the Two-Dimensional Limit

Probing Carrier Transport and Structure-Property Relationship of Highly Ordered Organic Semiconductors at the Two-Dimensional Limit

Observation of Frenkel and charge transfer excitons in pentacene single crystals using spectroscopic generalized ellipsometry

Precise Layering of Organic Semiconductors

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